The present invention relates to the art of molds for forming ingots from molten metal and, more particularly, to an improved mold plug, method of making a mold plug, and method for plugging and sealing the opening in the bottom of a top pouring ingot mold.
Top pouring, bottom plugged ingot molds are well known in the art of metal casting and are characterized by having an opening through the bottom wall thereof which is closed by a removable plug prior to the pouring of molten metal into the top of the mold to form an ingot. Follow:ing the casting operation, the plug is destructible to facilitate removal or stripping of the ingot from the mold by impacting a tool against the bottom of the ingot through the opening in the bottom wall of the mold.
Plugs for such ingot molds are most often molded or otherwise formed from ceramic, refractory or carbon materials, or combinations thereof, and are of inverted frusto-conical configuration to provide top and bottom ends and an outer peripheral surface corresponding generally in contour with that of the plug opening in the bottom of the ingot mold. The surface on the bottom wall of a mold about the plug opening and the surface of the plug opening through the bottom wall are generally clean and smooth when the mold is new or relatively new and, unless damaged prior to use or during insertion of the plug into the mold opening, the outer peripheral surface of the preformed plug is also generally smooth. Under such conditions, the preformed plugs are suitable for closing the plug opening so as to minimize leakage of molten metal from the mold during an ingot casting operation and, generally, satisfactorily provide their intended function. However, the bottom surface of the mold and the surface of the mold opening become irregular with repeated use of the mold, and such irregularity is progressive in nature. Irregularities are caused in part, for example, by erosion due to metal leakage, and by the cutting away by an oxygen torch of metal which becomes welded to the surface of the mold opening as a result of leakage. As a result of these irregularities, gaps exist between the mold opening and plug when the latter is introduced into the plug opening prior to a casting operation. Such gaps provide leakage paths for molten metal subsequently poured into the mold, and leakage is undesirable from the standpoint of lost metal, time lost in connection with maintenance requirements resulting from leakage, and the danger of potential injury to workmen exposed to the molten metal. When leakage becomes excessive, replacement of the mold becomes necessary. Such replacement is expensive and often occurs when the mold is otherwise still usable. Efforts heretofore to prolong the use of a leaking mold and to defer mold replacement have included the procedure of initially pouring a small amount of molten metal into the bottom of the mold and allowing it to fill the leakage paths and solidify to close the leakage paths prior to pouring the remainder of the molten metal into the mold to complete the casting operation. It will be appreciated that this procedure is undesirable from the standpoint of being time consuming and, moreover, does not overcome the disadvantage of lost metal through leakage. Moreover, this procedure can leave metal welded to the opening and which must be cut away as explained above, causing further deterioration and thus further reducing mold life.
Further problems encountered in connection with the use of bottom plugged top pouring ingot molds include the thermal welding of molten metal to the wall of the ingot mold itself. Such welding, upon removal of the cast ingot, causes side wall erosion in the mold, thus reducing mold life. Further, if such erosion becomes severe, the ingot can become lodged in the mold to the extent that it cannot be removed therefrom, whereby both the ingot and the mold have to be scrapped. This is of course expensive both from the standpoint of lost metal and mold replacement. Another problem, which is essentially independent of the condition of the opening in the bottom wall of the ingot mold, is that the mold plug often is not sufficiently lodged or secured in the plug opening whereby, upon pouring molten metal into the ingot mold, the plug floats to the top of the metal bath because of the difference in specific gravity between the molten metal and plug material. It will be appreciated that this problem results in a total loss of metal from the ingot mold, erosion of the bottom opening by the flow of molten metal therethrough, the thermal welding of metal to the surface of the opening, the destruction of equipment supporting the mold, such as a railway car and tracks therebeneath, and the potential injury to workmen near the mold and having no warning with respect to such release of molten metal. While such a floating plug problem may only be occasional in occurrence, it will be appreciated that it is potentially the most serious and dangerous problem from the standpoint of equipment deterioration and loss, and personal injury.